1. Field of the Invention
The present invention relates to a process for the preparation of a film essentially consisting of thermoplastic polyimide. More particularly, the invention relates to a preparation process of a thermoplastic polyimide film comprising cooling a molten film under specific conditions and taking-up the film under tension in an extrusion process.
2. Description of the Prior Art
Industrial machinery in various fields has recently been directed toward miniaturization and weight reduction. The parts for constituting the machinery are also required to have reliability over prolonged periods in the areas of heat resistance, mechanical properties and dimensional stability. Plastic films used for the above industrial machinery are also required to have the above performance.
At present, film materials having the highest heat-resistance designed to meet the above requirement are various polyimide films such as Upilex.TM. (a product of Ube Industries Co. Ltd.) and Kapton.TM. (a product of E. I. Du Pont De Nemours & Co.). Since hot molding of the above polyimide is difficult, films are prepared by a so-called solution casting method where a polyamic acid precursor solution is cast and heat-treated to form a polyimide film.
On the other hand, hot-moldable polyimides such as polyetherimide and polyimide which are disclosed in Japanese Patent Laid-Open Publication SHO No. 62-205124(1987) have been developed. Using the thermoplastic polyimide, films can be prepared by a so-called extrusion process where polyimide is heat-melted in an extruder and the molten polyimide delivered from a die is cooled solidified by contact with a cooling roller.
In order to obtain high quality film by an extrusion process, adhesion of the molten film to the cooling roller is important in the cooling and solidifying step of the extruded molten material from the die.
The methods for improving adhesion include:
(1) A method for blowing air out of a slit nozzle (air knife method).
(2) A method for mechanically pressing the molten film with a rubber roller at the contact point of the molten film with the cooling roller (rubber roller method).
(3) A method for applying static charge to the molten film and electrostatically influencing the film into close contact with the cooling roller (static charge application method).
However, in the extrusion process of the above thermoplastic polyimide, the air knife method has a drawback in that it is difficult to evenly provide the whole film with sufficient pressure to uniformly adhere the molten film to the cooling roller. Consequently, cooling becomes non-uniform and has caused problems such as defects in flatness and spot variation in the thickness of the solidified film.
As to the rubber roller method, for example, Japanese Patent Laid-Open Publication SHO No. 62-290515(1987) discloses a method for assuming the glass transition point (Tg) of the resin to be processed as the reference temperature, setting the surface temperature of the cooling roller in the range of (Tg)-50.degree. to (Tg)-10.degree. C., and nipping the film with a rubber roller having a surface hardness of 70 degrees or less under a specific range of linear pressure. However, the rubber roller deforms during use even though the roundness of the roller is extremely accurate in the initial stage, which leads to irregularity in linear pressure of the rubber roller and has caused problems such as spot variation in the thickness of the film.
As to the static charge application method, for example, Japanese Patent Publication HEI No. 1-53620(1989) describes a method for preparing an amorphous film having low optical strain by applying static charge to a molten film, uniformly adhering the film to a cooling roller having a temperature of the glass transition point (Tg)-10.degree. C. and transmitting the film without tension by using a second roller having a slightly lower peripheral speed then that of the cooling roller. The static charge application method provides good adhesion of the film to the cooling roller. In the method, however, it is difficult to maintain flatness of the film and to prevent generation of creases because electrostatically adhered film is released from the cooling roller without tension.
Further, the film obtained has been defective in that very small flaws are observed over the whole surface of the film.